The present invention relates to aluminum cylinder tubes for air cylinders, hydraulic cylinders or the like, for example, for use in industrial robots, vehicles and other industrial machines, and to a process for producing the same.
The term "aluminum" as herein used includes aluminum and aluminum alloys.
Generally, cylinder tubes of the type mentioned are formed with an abrasion-resistant hard film on the inner surface to withstand the reciprocation of pistons. Conventional cylinder tubes are produced by subjecting the inner and outer surfaces of an aluminum tube to anodic oxidation by the low-temperature sulfuric acid electrolytic process or electrolytic spontaneous color forming process to form a hard or superhard oxide film on the surfaces. Thus, the conventional cylinder tube has not only on the inner surface but also on the outer surface the hard to superhard oxide film which is usually of black to blackish brown or like dark color and which therefore impairs the appearance of the tube. Since it is impossible to change the color to a light color, the tube has the problem of being unable to meet requirements as to the design. The conventional tube has another problem in that it is expensive to make because the outside oxide film is as thick as the inside film and has a thickness of at least about 20 .mu.m.